Grain oriented electrical steel strips (Fe-Si) are typically industrially produced as strips having a thickness comprised between 0.18 and 0.50 mm and are characterised by magnetic properties variable according to the specific product class. Said classification substantially refers to the specific power losses of the strip subjected to given electromagnetic work conditions (e.g. P50Hz at 1.7 Testa, in W/kg), evaluated along a specific reference direction (rolling direction). The main utilisation of said strips is the production of transformer cores. Good magnetic properties (strongly anisotropic) are obtained controlling the final crystalline structure of the strips to obtain all, or almost all, the grains oriented to have their easiest magnetisation direction (the [001] axis) aligned in the most perfect way with the rolling direction. In practice, final products are obtained having the grains mean diameter generally comprised between 1 and 20 mm having an orientation centred around the Goss orientation ({110} [001]). The minor the angular dispersion around the Goss one, the better the product magnetic permeability and hence the lesser the magnetic losses. The final products having low magnetic losses (core losses) and high permeability have interesting advantages in terms of design, dimensions and yield of the transformers.
The first industrial production of the above materials was described by the U.S. Firm ARMCO at the beginning of the thirties (U.S. Pat. No. 1,956,559). Many important improvements have been since introduced in the production technology of grain oriented electrical strips. In terms both of magnetic and physical quality of products and of transformation costs and cycles rationalisation. All existing technologies exploit the same metallurgical strategy to obtain a very strong Goss structure in the final products, i.e. the process of oriented secondary recrystallisation guided by uniformly distributed second phases and/or segregating elements. The, non metallic, second phases and the segregating elements play a fundamental role in controlling (slowing down) the movement of grain boundaries during the final annealing which actuates the selective secondary recrystallisation process.
In the original ARMCO technology, utilising MnS as inhibitor of the grain boundaries movement, and in the subsequent technology developed by NSC, in which the inhibitors are mainly aluminium nitrides (AlN+MnS) (EP 8.385, EP 17.830, EP 202.339), a very important binding step common to both production processes is the heating of the continuously cast slabs (ingots, in old times), immediately before the hot rolling, at very high temperatures (around 1400° C.) for a time sufficient to guarantee a complete dissolution of sulphides and/or nitrides coarsely precipitated during the slab cooling after casting, to re-precipitate them in a very fine and uniformly distributed form throughout the metallic matrix of the hot rolled strips. Such a fine re-precipitation can be started and, completed, as well as the precipitates dimensions adjusted, during the process, in any case, however, before the cold rolling. The slab heating to said temperatures requires using special furnaces (pushing furnaces, liquid-slag walking-beam furnaces, induction furnaces) due to the ductility at high temperatures of the Fe-3% Si alloys and to formation of liquid slags.
New casting technologies of the liquid steel are intended to simplify the production processes to make them more compact and flexible and to reduce costs. One of said technologies is the “thin slab” casting, consisting in the continuous casting of slabs having the typical thickness of conventional already roughened slabs, apt to a direct hot rolling, through a sequence of slabs continuous casting, treating in continuous tunnel-furnaces to rise/maintain the temperature of slabs and finishing-rolling, down to coiled strip. The problems connected to the utilisation of said technique for grain oriented products mainly consist in the difficulty to maintain and control the high temperatures necessary to keep in solution the elements forming the second phase, which have to be finely precipitated at the beginning of the finishing hot-rolling step, if desired best micro-structural and magnetic characteristics are to be obtained in the end-products. Such problems were dealt with in different ways, for instance utilising the low thickness of the cast slabs in connection to specific concentration intervals of the micro-alloying elements to stably control the second phases precipitation (grain growth inhibitors) during hot rolling, or drastically modifying the strategy of the inhibitors formation in the metal matrix.
The casting technique potentially offering the highest rationalisation level of the processes and the higher production flexibility is the one consisting in the direct production of strips from the liquid steel (Strip Casting), totally eliminating the hot rolling step. Such an exaordinary innovation was conceived and patented long time ago, and since long time were also devised and patented process conditions to produce electrical steel strips, and more particularly grain oriented ones. However, up to now there is not an industrial production in the world of grain oriented electrical steel according to the above technique, though the state of the art relating to the casting machines is ready for industrial applications, as shown by existing plants, producing only carbon steels and stainless steels.
The present inventors believe that to industrially produce grain oriented electrical steel strips from direct solidification of a strip (Strip Casting) it is necessary to have a strip micro-structure before cold rolling significantly different from the one obtained during the casting stage. The high solidification speed of the cast strip makes it difficult to have a homogeneous and reproducible grain structure throughout the strip and between different castings, due to the high sensitivity of the solidification structure to the fluctuations of the casting conditions and to the alloy composition. The micro-structure of the intermediate products starting from strip casting is much more influenced by the solidification structure, with respect to the ones derived from conventional slab casting, because of the lack of deformation in the strip during the typical hot rolling.